Many telephone switching systems have idiosyncracies that can cause permanent trunk conditions. These permanent trunk conditions arise when two such connected telephone switching systems attempt to disconnect from each other. When a permanent trunk condition arises misdirected calls and no-ring conditions result.
Many telephone switching systems can initiate a new call in only 32 miliseconds after disconnecting from an associated switching system. However, many other telephone switching systems require substantially more time to disconnect. Therefore, if a first switching system initiates a new call 32 milliseconds after disconnecting from a second switching system which is slower to disconnect, the first switching system may forward a new call into a path that has not been released from a previous call.
An 850 millisecond tip-lead open condition is required between connected switching systems in order to ensure that the second switching system has completely disconnected from the first switching system before the first switching system attempts to forward a new call to the second switching system.
The typical approach for providing this 850 millisecond tip-lead open condition is to use C lead control signals. Such an arrangement does provide improved resolution of the disconnect problem, but use of C lead control signals causes problems in the engineering and administration areas of the telephone operating companies.
The present invention discloses uses an 850 millisecond disconnect circuit which is transparent to the telephone switching systems since it is installed in series with the tip and ring leads.